Process of forming ultra thin wafers having an edge support ring

ABSTRACT

A process of forming ultra thin wafers having an edge support ring is disclosed. The process provides an edge support ring having an angled inner wall compatible with spin etch processes.

CROSS REFERENCE TO RELATED APPLICATIONS

The instant patent application is a divisional patent application andclaims priority to U.S. patent application Ser. No. 11/880,455 filed onJul. 20, 2007, now U.S. Pat. No. 8,048,775 which is herein incorporatedby reference in its entirety and which claims priority to U.S. patentapplication Ser. No. 11/694,888 filed on Mar. 30, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to semiconductor devicefabrication and more particularly to a process of forming ultra thinwafers having an edge support ring.

2. Description of Related Art

Vertical power devices including power MOSFETs and insulated gatebipolar transistors (IGBTs) are being fabricated on ultra thin wafers onthe order of less than 4 mils. The fabrication of these power devices onultra thin wafers provides for devices with reduced electrical andthermal resistances. The use of ultra thin wafers further ensures thatthe power devices meet stringent total package thickness requirements.Furthermore, ultra thin wafers—including ultra thin float zonewafers—are replacing expensive wafers with an epitaxially formed siliconlayer for device junctions and buffer zones.

For ultra thin wafers with thickness smaller than 4 mils, wafer bowingand warping, and/or breakage and fracture occur frequently duringhandling processes. The prior art generally provides for two waferthinning and handling approaches. In a first approach, a temporarysupport substrate such as a handle wafer, a glass substrate, a thicktape, a polymer substrate, and a polymer based composite substrate isapplied. Disadvantageously, this first approach requires complexoperations when applying and removing the temporary support structure.Some removal processes may involve risk of wafer breakage and fracture.Additionally, adhesives and other polymeric materials used to secure thewafer to the temporary support structure may outgas in the vacuumchamber during metallization and adversely affect the quality of theohmic contact.

In a second approach, a support ring is formed at a wafer edge tofacilitate handling and processing of the ultra thin wafer. This secondapproach advantageously eliminates the introduction of extraneousmaterials such as adhesives into the processing of the ultra thin wafer.As disclosed in U.S. Pat. No. 6,162,702 entitled “Self-supported UltraThin Silicon Wafer Process”, and illustrated in FIGS. 1 and 2, a siliconwafer 2 has an ultra thin central portion 4 that is supported by acircumferential rim 3 of thicker silicon. Removing a volume of siliconfrom the silicon wafer 2 by a controlled mechanical or physical meanssuch as grinding, milling, drilling or laser forms the ultra thincentral portion 4. Alternatively, a mask may be formed on the rim of thesilicon wafer 2 and the silicon wafer 2 may be etched to form the ultrathin central portion 4.

With reference to FIGS. 3, 4 and 5, a grinding wheel 30 having teeth 32is shown for use in grinding a backside 34 of a wafer 31 and forming athick edge support ring 36 therearound. The grinding wheel 30 has anaxis of rotation “G” while the wafer 34 has an axis of rotation “W”.Grinding of the wafer backside 34 yields an ultra thin central portion38 having a thickness less than 4 mils surrounded by the edge supportring 36. The edge support ring is roughly the same thickness as theoriginal wafer. The edge support ring is at least 2 mm wide. The widerthe ring, the stronger it is, but also the less space is available toform dies. The edge support ring 36 facilitates the handling of thewafer 31.

Following the back grinding process, it is conventional to spin etch theultra thin central portion 38 of the wafer 31 before the backmetallization process. The spin etch process includes a chemical etch ofsilicon and oxides followed by cleaning with de-ionized water. The spinetch process increases the mechanical strength of the ultra thin centralportion 38 and ensures good ohmic contact between the metal subsequentlydeposited and the highly doped silicon substrate of the wafer 31.Following the spin etch process, no drying process is necessary beforeloading the wafer 31 into the vacuum chamber for back metallization.

It has been discovered that the structure of the edge support ring 36disadvantageously interferes with the spin etch process. As seen in FIG.6 and indicated by the arrows, rotation of the wafer 31 about the axisof rotation “W” causes the chemical etch and de-ionized water to flowoutwardly and encounter a wall 60 of the edge support ring 36 formed ata right angle or nearly a right angle to the plane of the ultra thincentral portion 38. The wall 60 prevents the entirety of the chemicaletch and de-ionized water from being spun from the wafer 31 during thespin etch process. As such, additional cleaning and/or drying steps arerequired before the back metallization process can be effectivelyperformed. Alternatively, the wafer 31 may be baked at a hightemperature to remove the residual chemical etch and de-ionized water.In either case unacceptable complexity or delays in the fabricationprocess may be introduced by the structure of the edge support ring 36of the prior art that additionally may adversely affect the quality ofthe back metal contact.

There remains a need in the art for a process of forming ultra thinwafers having an edge support ring that overcomes the limitations of theprior art. There is also a need in the art for a process of formingultra thin wafers having an edge support ring that provides for an edgesupport ring compatible for use with conventional spin etch processesbefore back metal deposition. There is a further need for an edgesupport ring having an angled inner wall.

SUMMARY OF THE INVENTION

The process of forming ultra thin wafers having an edge support ring inaccordance with a preferred embodiment of the invention provides a waferhaving an edge support ring surrounding an ultra thin central portion ofless than 4 mils thickness. The edge support ring has an angled innerwall extending from the ultra thin central portion to a top of the edgesupport ring. The angled inner wall provides a means by which thechemicals used in the spin etch process are spun from the wafer duringspin etch processing.

In accordance with one aspect of the invention, an ultra thin waferhaving an edge support ring includes an ultra thin central portion and aperipheral edge support ring, the edge support ring having an angledinner wall.

In accordance with another aspect of the invention, a process of formingan ultra thin wafer having an edge support ring includes the steps ofpositioning a grinding wheel proximate a wafer backside edge, grindingthe wafer backside while gradually reducing a distance between agrinding wheel axis of rotation and a wafer chuck axis of rotation, andending the grinding of the wafer backside when an ultra thin centralportion thickness is achieved.

In accordance with yet another aspect of the invention, a process offorming an ultra thin wafer having an edge support ring includes thesteps of positioning a grinding wheel having profiled teeth proximate awafer backside edge, grinding the wafer backside, and ending thegrinding of the wafer backside when an ultra thin central portionthickness is achieved.

In accordance with another aspect of the invention, a process of formingan ultra thin wafer having an edge support ring includes the steps ofpositioning a first grinding wheel a distance from a wafer backsideedge, grinding a wafer backside to form an ultra thin central portionand an edge support ring until an ultra thin central portion thicknessis achieved, positioning a second grinding wheel to grind an angledinner wall of the edge support ring, and grinding the edge support ringto form the angled inner wall.

There has been outlined, rather broadly, the more important features ofthe invention in order that the detailed description thereof thatfollows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described below andwhich will form the subject matter of the claims appended herein.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of functional components andto the arrangements of these components set forth in the followingdescription or illustrated in the drawings. The invention is capable ofother embodiments and of being practiced and carried out in variousways. Also, it is to be understood that the phraseology and terminologyemployed herein, as well as the abstract, are for the purpose ofdescription and should not be regarded as limiting.

As such, those skilled in the art will appreciate that the concept uponwhich this disclosure is based may readily be utilized as a basis forthe designing of other methods and systems for carrying out the severalpurposes of the present invention. It is important, therefore, that theclaims be regarded as including such equivalent constructions insofar asthey do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 is a top plan view of a prior art ultra thin wafer having an edgesupport ring;

FIG. 2 is cross sectional view of the ultra thin wafer taken along lineX-X′ of FIG. 1;

FIG. 3 is a schematic representation showing a grinding wheel for use ingrinding a central portion of a wafer;

FIG. 4 is a partial side elevation view showing the wafer being groundby the grinding wheel;

FIG. 5 is a partial side elevation view of the ground wafer including anultra thin central portion and an edge support ring;

FIG. 6 is a schematic representation showing the flow of spin etchchemicals during a spin etch process;

FIG. 7 is a schematic representation showing the flow of spin etchchemicals during a spin etch process in accordance with an aspect of theinvention;

FIG. 8 is a schematic representation showing the flow of spin etchchemicals during a spin etch process in accordance with another aspectof the invention;

FIG. 9 is a partial side elevation view showing a wafer being ground inaccordance with a first aspect of the invention;

FIG. 10 is a partial side elevation view showing the wafer of FIG. 9ground to include an edge support ring having an angled inner wall;

FIG. 11 is a partial side elevation view showing a wafer being ground inaccordance with a second aspect of the invention;

FIG. 12 is a partial side elevation view showing the wafer of FIG. 11ground to include an edge support ring having an angled inner wall;

FIG. 13 is a partial side elevation view showing a wafer being ground inaccordance with a third aspect of the invention;

FIG. 14 is a partial side elevation view showing the wafer of FIG. 13ground to include an edge support ring having an angled inner wall;

FIG. 15 is a partial side elevation view showing an ultra thin waferwith back metallization in accordance with the invention;

FIG. 16 is a flow chart of a process of forming ultra thin wafers havingan edge support ring in accordance with the first aspect of theinvention;

FIG. 17 is a flow chart of a process of forming ultra thin wafers havingan edge support ring in accordance with the second aspect of theinvention; and

FIG. 18 is a flow chart of a process of forming ultra thin wafers havingan edge support ring in accordance with the third aspect of theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The present invention will now be described in detail with reference tothe drawings, which are provided as illustrative examples of theinvention so as to enable those skilled in the art to practice theinvention. Notably, the figures and examples below are not meant tolimit the scope of the present invention. Where certain elements of thepresent invention can be partially or fully implemented using knowncomponents, only those portions of such known components that arenecessary for an understanding of the present invention will bedescribed, and detailed descriptions of other portions of such knowncomponents will be omitted so as not to obscure the invention. Further,the present invention encompasses present and future known equivalentsto the components referred to herein by way of illustration.

In accordance with a first embodiment of the invention and withreference to FIG. 7, a wafer 70 includes an ultra thin central portion71 and a peripheral edge support ring 75 formed on a backside 79thereof. A well-defined angle B is formed at a bottom of an angled innerwall 76 of the edge support ring 75 and an edge 72 of the ultra thincentral portion 71. Angled inner wall 76 extends upwardly from a planeof the ultra thin central portion 71 to a top 78 of the edge supportring 75. Chemical etch and de-ionized water can be spun from the wafer70 during the spin etch process as indicated by the arrows. The edgesupport ring 75 provides the benefits of reduced handling and processingof the wafer 70 during subsequent processing and provides a solution tothe problems described in the Background of the Invention sectionhereof.

In accordance with another embodiment of the invention and withreference to FIG. 8, a wafer 80 includes a substantially non-linearangled inner wall 86 extending and curving upwardly from a plane of anultra thin central portion 81 to a top 88 of an edge support ring 85formed on a backside 89 thereof. Chemical etch and de-ionized water canbe spun from the wafer 80 during the spin etch process as indicated bythe arrows. The edge support ring 85 provides the benefits of reducedhandling and processing of the wafer 80 during subsequent processing andprovides another solution to the problems described in the Background ofthe Invention section hereof.

In accordance with a first aspect of a process of the invention, aprocess of forming ultra thin wafers having an edge support ringgenerally designated 160 is shown in FIG. 16. The process 160 uses agrinding wheel having an axis of rotation and a wafer chuck to which issecured the wafer, the wafer chuck having an axis of rotation. In apreferred embodiment of the invention, the axes of rotation of thegrinding wheel and the wafer chuck are parallel. In a step 163 thegrinding wheel is positioned proximate an edge of a wafer backside. Bystarting proximate the edge of the wafer, wafer material at the edge ofthe wafer is provided to form an edge support ring of substantially thesame thickness of the original wafer and at least 2 mm wide. In a step165 the wafer backside is ground while gradually reducing the distancebetween the grinding wheel axis of rotation and the wafer chuck axis ofrotation. The decrease in the distance may be a function of the feeddepth of the grinding wheel or a function of time. In a final step 167,grinding of the wafer backside is stopped when an ultra thin centralportion thickness is achieved.

With reference to FIG. 9, a wafer 90 having a backside 93 is shown beingground by a grinding wheel 95 in accordance with the process 160. Anedge support ring 97 and an angled inner wall 99 are shown being formedas the wafer backside 93 is ground and as the distance between thegrinding wheel axis of rotation and the wafer chuck axis of rotation isgradually reduced. As shown in FIG. 10, the wafer 90 having an ultrathin central portion 100 and an edge support ring 97 having an angledinner wall 99 are formed by the process 160.

In accordance with a second aspect of a process of the invention, aprocess of forming ultra thin wafers having an edge support ringgenerally designated 170 is shown in FIG. 17. The process 170 uses agrinding wheel having an axis of rotation and a wafer chuck to which issecured the wafer, the wafer chuck having an axis of rotation. In apreferred embodiment of the invention, the axes of rotation of thegrinding wheel and the wafer chuck are parallel. The grinding wheel hasa plurality of teeth having an angled profile at an outside surfacethereof. The angled profile preferably corresponds to a desired profileof an edge support ring angled inner wall. In a step 173, the grindingwheel is positioned proximate an edge of a wafer backside. By startingproximate the edge of the wafer, wafer material at the edge of the waferis provided to form an edge support ring of substantially the samethickness of the original wafer and at least 2 mm wide. In a step 175,the wafer backside is ground forming the edge support ring having theangled inner wall. The process 170 ends in a step 177 when an ultra thincentral portion thickness is achieved. After grinding many wafers, theteeth of the grinding wheel may become worn, necessitating an adjustmentof the distance between the grinding wheel axis of rotation and thewafer chuck axis of rotation in step 173.

A grinding wheel 110 for use in the process 170 is shown in FIG. 11 andincludes teeth 111 having an angled profile 112 at an outside surface113 thereof. The angled profile 112 preferably corresponds to thedesired profile of the angled inner wall 114 of the edge support ring115 as shown in FIG. 12.

In accordance with a third aspect of a process of the invention, aprocess of forming ultra thin wafers having an edge support ringgenerally designated 180 is shown in FIG. 18. The process 180 uses twogrinding wheels to form the edge support ring having an angled innerwall. In a first step 183, a first grinding wheel is positioned adistance from an edge of the wafer. In a preferred embodiment of theinvention, the axes of rotation of the first grinding wheel and thewafer chuck are parallel. By starting a distance from the edge of thewafer, wafer material at the edge of the wafer is provided to form theedge support ring. In a step 185, a wafer backside is ground until theultra thin central portion thickness is achieved. At this stage of theprocess 180, a wafer 130 having been ground using a first grinding wheel132 includes an ultra thin central portion 131 and an edge support ring133 having a substantially perpendicular wall 135 as shown in FIG. 13.Following step 185, a second grinding wheel is positioned at an angle tothe substantially perpendicular wall to grind an angled inner wall ofthe edge support ring in a step 187. The angled inner wall is thenground in a step 189. The wafer 130 being ground by a second grindingwheel 140 is shown in FIG. 14. Grinding the perpendicular wall 135 formsan angled inner wall 141.

Processes 160, 170 and 180 may be employed to provide an angled innerwall having a linear profile such as that of angled inner wall 76 (FIG.7). Processes 160 and 170 may be employed to provide a non-linearprofile such as that of angled inner wall 86 (FIG. 8). Following theformation of the edge support ring having an angled inner wall inaccordance with the invention, the wafer may be spin etched and thebackside thereof metalized to provide an ultra thin wafer 150 having aback metal 153 as shown in FIG. 15.

The process of forming ultra thin wafers having an edge support ring inaccordance with the invention provides ultra thin wafers having edgesupport rings compatible for use with conventional spin etch processes.The angled inner wall of the edge support ring provides a means by whichetch chemicals and de-ionized water is spun from the wafer backside tothereby prepare the wafer for back metallization.

It is apparent that the above embodiments may be altered in many wayswithout departing from the scope of the invention. Further, variousaspects of a particular embodiment may contain patentably subject matterwithout regard to other aspects of the same embodiment. Still further,various aspects of different embodiments can be combined together.Accordingly, the scope of the invention should be determined by thefollowing claims and their legal equivalents.

What is claimed is:
 1. A process of forming an ultra thin wafer havingan edge support ring comprising the steps of: positioning a grindingwheel proximate a wafer backside edge; grinding a periphery of the waferbackside while gradually reducing a distance between a grinding wheelaxis of rotation and a wafer chuck axis of rotation, wherein thegrinding wheel axis of rotation is parallel to the wafer chuck axis ofrotation; gradually decreasing feed depth of said grinding wheel as afunction of said decrease in said distance between said wheel axis ofrotation and said wafer chuck axis of rotation to create an ultra thincentral portion and a peripheral edge support ring with an angled innerwall; and ending the grinding of the wafer backside when an ultra thincentral portion thickness is achieved.
 2. The process of forming theultra thin wafer of claim 1, wherein the ultra thin central portion hasa thickness of less than 4 mils.